1. Field of the Invention
The present invention relates to a semiconductor device including a plurality of kinds of MOS transistors and a method of manufacturing the same.
2. Description of the Related Art
The performance of an LSI is being rapidly improved in accordance with the miniaturization of the device, the improvement in the device performance achieved by the miniaturization, and the improvement in the degree of integration, which are achieved by the development of the process technology. The shortening in the wavelength of the light used in performing a pattern transfer in the lithography process for transferring a pattern onto mainly a semiconductor device greatly contributes to the miniaturization in the size of the device. The shortening in the wavelength of the light is performed by using successively a g-line (436 nm) of an Hg lamp, an i-line (365 nm) of an Hg lamp, a KrF laser (248 nm) and an ArF laser (193 nm). Also, when it comes to a fine design not larger than 0.1 xcexcm, a light exposure technology using an F2 laser having a further shorter wavelength and a direct image depicting technology using an X-ray or an electron beam are being studied.
However, the development of the light exposure technology and the image depicting technology using an optical system and an electronic system utilizing light of a shorter wavelength necessitates an improvement in the mechanical accuracy for realizing the miniaturization so as to make it more difficult technically to develop the particular light exposure technology and the image depicting technology.
Under the circumstances, the resolution limit has been improved by improvements in the resist material on to which a pattern is transferred, and by improvements in the illuminating method in the lithography process. Also, it is possible nowadays to form a device of a size not larger than half the wavelength of the light by an ultra high resolution technology called xe2x80x9cphase shift methodxe2x80x9d in which the contrast of the light on a wafer is increased by controlling the phase of the light for each pattern on a reticle.
However, in the phase shift method, the interference of light needs to be skillfully performed to achieve a high contrast, which makes it difficult to form the smallest device size uniformly in respect of various pattern arrangements.
An ultra high resolution light exposure technology, achieved by the xe2x80x9cLevenson methodxe2x80x9d, which is a kind of a phase shift method, will now be described. In the Levenson method, a shifter is arranged such that the phases of the light on both sides of a pair of patterns on a reticle (mask) are reversed to each other so as to improve the contrast. Since the contrast is enhanced in the Levenson method by allowing light beams substantially equal to each other in intensity to interfere with each other, the Levenson method is one of the best methods offered by ultra high resolution technology.
However, if there is a boundary portion in the shifter, the light interferes with the light of the reversed phase even in the region in which a pattern is not formed, with the result that a resist pattern remains, as in the pattern region. In order to avoid this difficulty, in the Levenson method the final pattern formation is generally performed using a reticle called a xe2x80x9ctrim maskxe2x80x9d, in addition to an xe2x80x9calternate mask (Alt mask)xe2x80x9d having a shifter formed therein in advance.
FIGS. 1A and 1B exemplify an Alt mask and a trim mask used for forming the gate electrode of a MOS transistor. In the Alt mask shown in FIG. 1A, a light shielding portion 61 consisting of, for example, a Cr film is formed on a quartz plate. A shifter is formed in an open portion 62 in which the Cr film is not formed by removing the quartz plate in that portion by a predetermined thickness. In the trim mask shown in FIG. 1B, the light shielding portion 61 alone consisting of a Cr film is formed on the quartz plate.
FIG. 1C shows an overlay image in which the Alt mask and the trim mask are overlapped with each other. FIG. 1D shows a pattern obtained by exposing a resist to light by overlapping the Alt mask and the trim mask, followed by applying a developing treatment. In FIG. 1D, a resist pattern 63 is used for patterning the gate electrode. Reference numeral 64 shown in FIG. 1D denotes an active region of the MOS transistor.
The specific method of manufacturing a memory mixed LSI in which an inner logic circuit and a DRAM circuit are formed integral by the Levenson method will now be described with reference to FIGS. 2A, 2B to 5A, 5B. Incidentally, FIGS. 2A to 5A are cross sectional views each showing a part of the inner logic circuit. Also, FIGS. 2B to 5B are cross sectional views each showing a part of the DRAM circuit. In the DRAM circuit, the STI (shallow trench isolation) structure, the trench capacitor structure, etc. are omitted, and the construction of the gate electrode alone is shown in the drawings.
In the first step, as shown in FIGS. 2A and 2B, a p-well region 72 and an n-well (not shown) are formed on a semiconductor substrate 71 in order to form a CMOS transistor. Then, after formation of an isolation insulating film 73, gate insulating films 74, 75 are formed on the side of the inner logic circuit and on the side of the DRAM circuit, respectively. Further, a polycrystalline silicon (polysilicon) film 76 is formed on the entire surface, followed by successively forming an antireflection film 77 and a resist film 78 on the polysilicon film 76.
In the next step, the gate electrode pattern is transferred into the resist 78 by a lithography process by the Levenson method using two masks, i.e., an Alt mask and a trim mask, followed by a developing treatment so as to leave the resist 78 unremoved in the shape as shown in FIGS. 3A and 3B.
Then, the antireflection film 77 is selectively removed by an anisotropic dry etching by using the resist 78 as a mask, as shown in FIGS. 4A and 4B. Further, a trimming treatment for reducing the sizes of the antireflection film 77 and the resist 78 is performed by isotropic etching, as shown in FIGS. 5A and 5B, followed by selectively etching the polysilicon film 76 by using the antireflection film 77 and the resist 78 as a mask so as to form a gate electrode 79 made of a polysilicon film in each of the inner logic circuit and the DRAM circuit. A standard CMOS process is employed in the subsequent step so as to form an LSI.
FIGS. 6A and 6B show the overlay images formed by the Alt mask and the trim mask used in the method described above. FIG. 6A shows the image on the side of the inner logic circuit, with FIG. 6B showing the image on the side of the DRAM circuit.
In FIG. 6A, a region 81 corresponds to the open portion on the Alt mask, and a region 82 corresponds to an open portion on the trim mask. Also, in FIG. 6B, a region 83 corresponds to an open portion on the trim mask. Incidentally, a reference numeral 84 shown in FIG. 6A denotes an active region of the MOS transistor.
In the conventional method described above, it is possible to form a fine gate electrode on the side of the DRAM circuit. However, it is necessary to ensure an allowance in the pattern clearance in view of the trimmed portion because a trimming treatment is also applied to the resist pattern on the side of the DRAM circuit.
Also, on the side of the inner logic circuit, it is certainly possible to form a fine gate electrode. However, it is necessary to design the pitch of the gate electrodes in a manner to have an allowance as in the design on the side of the DRAM circuit, with the result that a problem remains unsolved in terms of miniaturization of the device.
It should be noted that, in order to improve the performance of the logic LSI, it is strongly required to improve the device performance achieved by the miniaturization of the MOS transistor. In order to realize a complex logic in the logic LSI, the pattern arrangement of the MOS transistor is rendered more diversified, compared with the memory SLI. Further, because a complex wire connection is required for realizing various logic modes, the degree of integration of the MOS transistor is rendered lower than that of the memory LSI. Because of the low degree of integration and the necessity of forming fine the gate electrode, employed is the trimming technology for making thinner the resist formed by the lithography and the material deposited on the gate electrode by the isotropic etching as shown in FIGS. 5A and 5B so as to make it possible to reduce the final size of the gate electrode to a region not larger than one third of the wavelength of the light.
On the other hand, the progress in recent years of fine processing technology for producing semiconductor process has realized improvements in the device performance and an in the degree of integration. In recent years, the placing of a memory of a large capacity on a logic LSI has come the key technology in the improvement of the system performance. The strong demands for the mixed LSI satisfies simultaneously the various needs of devices differing from each other in the power source voltage on the LSI, and the processing technology of a fine pitch for realizing a large capacity memory.
In considering the system-on-silicon (SOC) for the future, it is necessary to form fine the gate electrode in the fine MOS transistor included in the inner circuit, required for a high operating speed. Also, it is necessary to increase the gate width of the gate electrode to some extent in the MOS transistor used in the peripheral circuit or in the memory circuit. However, the wiring pitch in the peripheral circuit or the memory circuit is at least as severe as that in the inner circuit. Also, the fine MOS transistor included in the inner circuit requiring a high operating speed differs from the MOS transistor used in the peripheral circuit or the memory circuit in the power source voltage used and in the severe retention performance required of the memory circuit. Under the circumstances, it is necessary to render these two MOS transistors different from each other in the thickness of the gate post oxide film and in the degree of making the corner portion roundish in the lower portion of the gate electrode.
However, in the conventional method, the patterning of the gate electrodes of all the MOS transistors is performed simultaneously. In addition, the trimming of the mask materials used for the patterning of the gate electrodes of all the MOS transistors is also performed simultaneously. It follows that it is difficult to freely set the optimum gate width, wiring pitch and thickness of the gate post oxide film of the gate electrode to suit the various intended uses of the gate electrode.
As described above, the prior art gives rise to the problem that it is difficult freely to set the optimum gate width, wiring pitch and thickness of the gate post oxide film to suit the various intended uses of the gate electrode, for integrating various MOS transistors.
According to one aspect of the present invention, there is provided a semiconductor device, comprises a first MOS transistor including a first gate electrode having a first gate width and a first gate post oxide film formed on a circumferential side wall of the first gate electrode; and a second MOS transistor including a second gate electrode having a second gate width smaller than the first gate width and a second gate post oxide film formed on a circumferential side wall of the second gate electrode and having a portion differing in thickness from the first gate post oxide film.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprises forming a gate insulating film in each of a region in which a first MOS transistor is to be formed and another region in which a second MOS transistor is to be formed on a semiconductor substrate; forming a film of a gate electrode material on the entire surface; forming a film of a resist material on the entire surface; patterning the film of the resist material in a manner to have a pattern of a size larger than the gate width of the gate electrode of at least the first MOS transistor in the region where the first MOS transistor is to be formed and to have a pattern conforming with the gate electrode of the second MOS transistor in the region in which the second MOS transistor is to be formed so as to form a first mask; selectively etching the film of the gate electrode material by an etching method using the first mask so as to form the gate electrode of the second MOS transistor; after removing the first mask, applying an oxidizing treatment so as to form a first gate post oxide film on a circumferential side wall of the gate electrode of at least the second MOS transistor; after forming a film of a resist material on the entire surface, patterning the film of the resist material in a manner to have a pattern conforming with the gate electrode of the first MOS transistor in the region where the first MOS transistor is to be formed and a pattern covering the entire surface in the region in which the second MOS transistor is to be formed so as to form a second mask; selectively etching the film of the gate electrode material by an etching method using the second mask so as to form the gate electrode of the first MOS transistor; and applying an oxidizing treatment so as to form a second gate post oxide film differing in thickness from the first gate post oxide film on a circumferential side wall of the gate electrode of the first MOS transistor in the region other than the region in which the first gate post oxide film is formed.